Double Crystallization and Phase Separation in Polyethylene—Syndiotactic Polypropylene Di-Block Copolymers

Crystallization and phase separation in the melt in semicrystalline block copolymers (BCPs) compete in defining the final solid state structure and morphology. In crystalline–crystalline di-block copolymers the sequence of crystallization of the two blocks plays a definitive role. In this work we show that the use of epitaxial crystallization on selected crystalline substrates allows achieving of a control over the crystallization of the blocks by inducing crystal orientations of the different crystalline phases and a final control over the global morphology. A sample of polyethylene-block-syndiotactic polypropylene (PE-b-sPP) block copolymers has been synthesized with a stereoselective living organometallic catalyst and epitaxially crystallized onto crystals of two different crystalline substrates, p-terphenyl (3Ph) and benzoic acid (BA). The epitaxial crystallization on both substrates produces formation of highly ordered morphologies with crystalline lamellae of sPP and PE highly oriented along one direction. However, the epitaxial crystallization onto 3Ph should generate a single orientation of sPP crystalline lamellae highly aligned along one direction and a double orientation of PE lamellae, whereas BA crystals should induce high orientation of only PE crystalline lamellae. Thanks to the use of the two selective substrates, the final morphology reveals the sequence of crystallization events during cooling from the melt and what is the dominant event that drives the final morphology. The observed single orientation of both crystalline PE and sPP phases on both substrates, indeed, indicates that sPP crystallizes first onto 3Ph defining the overall morphology and PE crystallizes after sPP in the confined interlamellar sPP regions. Instead, PE crystallizes first onto BA defining the overall morphology and sPP crystallizes after PE in the confined interlamellar PE regions. This allows for discriminating between the different crystalline phases and defining the final morphology, which depends on which polymer block crystallizes first on the substrate. This work also shows that the use of epitaxial crystallization and the choice of suitable substrate offer a means to produce oriented nanostructures and morphologies of block copolymers depending on the composition and the substrates.

Micellar Assembly and Disassembly of Organoselenium Block Copolymers through Alkylation and Dealkylation Processes

The aim of this work is to demonstrate that the alkylation and dealkylation of selenium atoms is an effective tool in controlling polymer amphiphilicity and, hence, its assembly and disassembly process in water. To establish this concept, poly(ethylene glycol)-block-poly(glycidyl methacrylate) was prepared. A post-synthesis modification with phenyl selenolate through a base-catalyzed selenium-epoxy ‘click’ reaction then gave rise to the side-chain selenium-containing block copolymer with an amphiphilic character. This polymer assembled into micellar structures in water. However, silver tetrafluoroborate-promoted alkylation of the selenium atoms resulted in the formation of hydrophilic selenonium tetrafluoroborate salts. This enhancement in the chemical polarity of the second polymer block removed the amphiphilic character from the polymer chain and led to the disassembly of the micellar structures. This process could be reversed by restoring the original amphiphilic polymer character through the dealkylation of the cations.

Physical Property Scaling Relationships for Polyelectrolyte Complex Micelles

<p>Polyelectrolyte complex micelles (PCMs) are widely used in the delivery of hydrophilic payloads. Their attractive features include an ability to tune physical attributes, which are strongly dependent on the size and chemical structure of each polymer block. Neutral blocks drive nanoscale phase separation while charged blocks control micelle core size and stability. An understanding of physical property behavior controlled by block size is crucial when designing for use in dynamic or biological environments and provides a greater understanding of the physics of polyelectrolyte assembly. In this work, we use small angle x-ray scattering, and light scattering to determine precise scaling behaviors of physical micelle parameters for commonly used polyelectrolytes. We then compare our results to accumulated published data and theory to show strong agreement, suggesting these laws are universal for PCMs.</p>

Physical Property Scaling Relationships for Polyelectrolyte Complex Micelles

<p>Polyelectrolyte complex micelles (PCMs) are widely used in the delivery of hydrophilic payloads. Their attractive features include an ability to tune physical attributes, which are strongly dependent on the size and chemical structure of each polymer block. Neutral blocks drive nanoscale phase separation while charged blocks control micelle core size and stability. An understanding of physical property behavior controlled by block size is crucial when designing for use in dynamic or biological environments and provides a greater understanding of the physics of polyelectrolyte assembly. In this work, we use small angle x-ray scattering, and light scattering to determine precise scaling behaviors of physical micelle parameters for commonly used polyelectrolytes. We then compare our results to accumulated published data and theory to show strong agreement, suggesting these laws are universal for PCMs.</p>

FRACTURE SIMULATION IN POLYMER NANOCOMPOSITES USING MOLECULAR DYNAMICS

The objective of this paper is to (a) investigate the validity of application of continuum-based linear elastic fracture mechanics (LEFM) methodology, which is often employed by researchers to model fracture processes at the “discrete” atomic scale, and (b) to study the effect of nanographene platelet size on the rupture strength of an edge-cracked polymer block. The material selected for this study is EPON 862 epoxy polymer with 85% cross-link density. Further, an atomistic J-integral is implemented as a nano-scale fracture metric to investigate flaw-tolerance at the nanoscale reported by many researchers, and to develop a methodology to predict the initiation fracture toughness of the material. For this purpose, a bond-order based potential (ReaxFF) available in LAMMPS , a molecular dynamics (MD) software, is utilized. Predictions obtained using the atomistic J-integral are compared with LEFM predictions for the case of a cross-linked epoxy polymer block with a center-crack under uniform far-field loading. Significant deviations from LEFM for crack-lengths below a certain critical crack-length threshold are observed. Further, far-field stress vs. strain plots are obtained for an edge-cracked epoxy polymer block with a single 14 nm graphene nanoplatelet embedded ahead of the crack tip and it is compared with stress vs. strain plot obtained for the same epoxy block with two 7 nm graphene nanoplatelets embedded ahead of the crack tip to study platelet size effect. Significant size effect was observed as shown in the results.

Secondary structure drives self-assembly in weakly segregated globular protein–rod block copolymers

Imparting secondary structure to the polymer block can drive self-assembly in globular protein–helix block copolymers, increasing the effective segregation strength between blocks with weak or no repulsion.